It used to be that a runner could tell when her running shoes were due for replacement just by seeing that the soles were worn away, the uppers were frayed and that the shoes were, generally, in pretty bad shape. With the improved materials used in today's shoes, however, it has become more difficult to visibly determine when a pair of running shoes has reached the end of its useful lifetime.
The materials used in the soles of running shoes today are so durable that, under most circumstances, they do not show much wear. Thus, the visible condition of the soles is no longer an accurate indication of a shoe's condition, especially where the concern is regarding the amount of cushioning that the shoe will continue to provide.
A running shoe, therefore, may look like it still has a few more miles in it when, in fact, the midsole has become so compressed that the shoe no longer provides a sufficient amount of cushioning for the user. Thus, the concern for a runner is to be able to tell when a pair of running shoes needs to be replaced.
As it is difficult to tell from a visual inspection when a pair of running shoes needs to be replaced, runners either have to be diligent in keeping records regarding the number of miles on each pair of shoes or wait for their knees and back to become so sore that it must be clear that the shoes need to be replaced. While there are certainly many runners out there who keep records for each pair of shoes as to the number of miles, the conditions the miles were run under, the times, etc., it is still not necessarily an accurate indication as to the actual conditions of the shoes. Sadly, many runners continue to use a pair of running shoes well past the point that would be recommended as healthy and only replace the shoes when they experience initial pain symptoms indicating personal physical degradation/damage, and/or can no longer tolerate the discomfort experienced typically in critical joint areas such as the knees and ankles.
What is needed, therefore, is an indicator to show how much cushioning capability is left in a shoe and to indicate to a user when a shoe needs to be replaced because it is not providing, or shortly will no longer provide, the appropriate amount of cushioning and/or energy rebound.